The present invention relates to a photoelectric conversion device applied to photographing equipment such as a still camera, video camera, and the like, various observation apparatuses, and the like, its control method, a focus detection device, and a storage medium which computer-readably stores processing steps of implementing the control method of the photoelectric conversion device and focus detection device.
Conventionally, various types of so-called auto-focus (AF) cameras, which detect the focus state of an object, and automatically focus on the object by changing the moving distance of the photographing lens in correspondence with the detected focus state, have been proposed.
Such AF cameras and the like use the method of detecting the focus state by, e.g., forming an object image on a photoelectric conversion element (to be referred to as a sensor hereinafter) formed by a plurality of photoelectric conversion pixels (to be simply referred to as pixels hereinafter), and performing predetermined arithmetic processing for a plurality of pixel signals output from the sensor.
In this method, in order to accurately detect the focus states of objects having various luminance levels (e.g., from a high-luminance object to low-luminance one), the amplification factor (to be referred to as a gain hereinafter) upon reading signals, and the charge accumulation time of the sensor must be appropriately controlled.
This is because if the level of an image signal of an object formed by a plurality of pixel signals (to be referred to as a video signal hereinafter) is too high, it exceeds the dynamic range of a pixel signal that can be processed by the apparatus, and the video signal becomes different from an actual one, thus impairing precision. By contrast, if the level of the video signal is too low, noise components increase relatively, and may impair precision.
FIG. 8 shows a photoelectric converter 500 which controls the read gain of pixel signals and the charge accumulation time in a sensor 54.
This photoelectric converter 500 comprises a sensor 54 constructed by a plurality of pixels, a peak detection circuit 53 for detecting and outputting a maximum accumulated charge amount during charge accumulation on the sensor 54, a memory 52 for receiving and holding pixel signals upon completion of charge accumulation on the sensor 54, a counter 55, a level output circuit 56 for outputting a level value selected from a plurality of level values in accordance with the count value of the counter 55, a comparator 57 for comparing the outputs from the level output circuit 56 and peak detection circuit 53, and outputting the comparison result, and a read amplifier 58 for outputting the pixel signals held in the memory 52 with the gain corresponding to the count value of the counter 55.
Note that the respective units of the photoelectric converter 500 are controlled by a controller 51, which especially controls charge accumulation on the sensor 54.
More specifically, as shown in FIG. 9, the controller 51 outputs a reset signal rst to the sensor 54 and counter 55 (step S501).
In response to this signal, charges on all the pixels of the sensor 54 are initialized, and the counter 55 is reset to an initial value “0” (count=0).
After that, charge accumulation on the sensor 54 is actually started.
Subsequently, the controller 51 sets its internal timer at an initial value “0” (timer=0), thus starting time measurement of the charge accumulation (step S502).
The controller 51 checks if the timer value timer of the internal timer has exceeded a maximum accumulation time Etime (step S503).
If “timer≧Etime”, the controller 51 determines the end of charge accumulation, and outputs a signal trans indicating this to the sensor 54. In response to this signal, charges accumulated on the individual pixels of the sensor 54 are transferred as pixel signals to the memory 52, thus ending charge accumulation on the sensor 54 (step S508).
On the other hand, if “timer<Etime” in step S503, the controller 51 checks if an output signal comp from the comparator 57 is “1”, i.e., if an output signal c_level of the level output circuit 56 is larger than an output signal p_out of the peak detection circuit 53 (step S504).
If “comp≠1”, the flow returns to step S503 to repeat the subsequent processing steps.
Note that the output signal c_level of the level output circuit 56 will be described in detail later.
If “comp=1” in step S504, the controller 51 checks if the internal timer value timer has exceeded an intermediate accumulation time Htime (step S505).
As a result of checking, if “timer≧Htime”, the flow advances to step S508, thus ending charge accumulation on the sensor 54.
However, if “timer<Htime” in step S505, the controller 51 checks if the count value count of the counter 55 is “3” (step S506).
If “count=3”, the flow advances to step S508, thus ending charge accumulation on the sensor 54.
On the other hand, if “count≠3” in step S506, the controller 51 outputs a signal up_c to the counter 55. In response to this signal, the count value count of the counter 55 is counted up (step S507).
After that, the flow returns to step S503 to repeat the subsequent processing steps.
Charge accumulation control of the sensor 54 is done in this way, and the read of pixel signals held in the memory 52 after completion of charge accumulation is controlled by a signal shift output from the controller 51.
With this control, pixel signals s_out read out from the memory 52 are multiplied by the gain by the read amplifier 58, and are output from an output terminal Vout.
At this time, the read amplifier 58 multiplies the pixel signals s_out from the memory 52 by the gain in accordance with the count value count of the counter 55.
The charge accumulation time of the sensor 54 is controlled by switching the output signal c_level of the level output circuit 56.
The charge accumulation time and the output signal c_level of the level output circuit 56 will be described below with reference to FIGS. 10A and 10B.
In the following description, assume that the level output circuit 56 has four level values “level1.0” to “level1.3”, and selectively outputs one of these level values in accordance with the count value count of the counter 55.
In FIGS. 10A and 10B, the abscissa plots the charge accumulation time, and the ordinate plots the values of the output signal c_level of the level output circuit 56 and the output signal p_out of the peak detection circuit 53.
FIG. 10A shows a case wherein the object is relatively bright, and the peak output of each pixel signal, i.e., the output signal p_out of the peak detection circuit 53 rises quickly. FIG. 10B shows, contrary to FIG. 10A, a case wherein the object is relatively dark, and the peak output of each pixel signal rises slowly.
(Case of FIG. 10A)
When charge accumulation is started, since the count value count of the counter 55 is initialized (step S501), the output signal c_level of the level output circuit 56 changes to “level1.0”.
When the charge accumulation time (timer value timer of the internal timer) has reached “A-1”, the output signal p_out of the peak detection circuit 53 exceeds the output signal c_level of the level output circuit 56. As a result, when the output signal comp of the comparator becomes “1”, the count value count of the counter 55 is counted up (steps S503 to S507). Since the counted-up count value count is supplied to the level output circuit 56, the output signal c_level of the level output circuit 56 changes to “level1.1”.
Similarly, when the charge accumulation time has reached “A-2”, the count value count of the counter 55 is counted up, and the output signal c_level of the level output circuit 56 changes to “level1.2”.
Also, when the charge accumulation time has reached “A-3”, the count value count of the counter 55 is counted up, and the output signal c_level of the level output circuit 56 changes to “level1.3”.
When the charge accumulation time has reached “A-4”, since the count value count of the counter 55 is “3”, charge accumulation on the sensor 54 ends (the flow advances to step S508 as a result of checking in step S506).
(Case of FIG. 10B)
When the charge accumulation time has reached “B-1” and “B-2”, the count value count of the counter 55 is counted up, and the output signal c_level of the level output circuit 56 changes from “level1.0” to “level1.1” and from “level1.1” to “level1.2”, in the same manner as in “A-1” to “A-3” mentioned above.
When the charge accumulation time has reached “B-3”, if it has exceeded the intermediate accumulation time due to the slowly rising output signal p_out of the peak detection circuit 53, charge accumulation on the sensor 54 ends (the flow advances to step S508 as a result of checking in step S506).
In this way, by switching the output signal c_level of the level output circuit 56 among four levels, the charge accumulation time is controlled in correspondence with the object condition, e.g., so that a sufficiently long charge accumulation time is assured when the object is light, or the charge accumulation time is prevented from becoming excessively long when the object is dark.
The gain of the read amplifier 58 is controlled in accordance with the count value count of the counter 55, and as a consequence, since the gain of the read amplifier 58 is controlled in accordance with the peak output (p_out) of each pixel signal, pixel signals can always be read out while effectively using the dynamic range of pixel signals that can be processed by the apparatus.
However, when the aforementioned conventional photoelectric converter 500 is applied to a multi-point AF camera which can effect the AF function at a plurality of distance measurement points, the arrangement including the comparator 57 and the like shown in FIG. 8 must be provided for each of all the distance measurement points. As a result, the circuit scale becomes huge, and the area of an IC chip increases.
In order to solve such problem, a method of dividing a single sensor into regions in units of distance measurement points, and controlling the charge accumulation time by a single controller while sequentially scanning the respective regions is proposed.
With this method, multi-point AF can be realized by a reasonable chip size while suppressing an increase in IC chip area.
However, in this method, when a pixel signal is read out from each region and is then compared to control the charge accumulation time of the region (sensor) of each distance measurement point, it is intermittently checked for a certain region during charge accumulation if charge accumulation is to end.
When such method is used in the photoelectric converter 500 shown in FIG. 8, since the output signal c_level of the level output circuit 56 is “level1.0” immediately after the beginning of charge accumulation, the count value count of the counter 55 becomes “3” for a high-luminance object which makes the output signal p_out of the peak detection circuit 53 rise rapidly, and charge accumulation ends. For this reason, much time is required, and the charge accumulation time cannot be appropriately controlled. As a result, since the level of the video signal of an object exceeds the dynamic range, the obtained image may be distorted. Also, in recent AF cameras, since the number of points for detecting the focus state (to be referred to as distance measurement points hereinafter) in the frame gradually is increasing like 3, 4, 5, . . . , the photographer need not change framing upon photographing after he or she sets a principal object in the frame at each distance measurement point and then focuses on the principal object, thus improving operability.
In order to further improve operability, the number of distance measurement points is preferably increased.
On the other hand, the focus state at each distance measurement point is detected by forming an object image on a photoelectric conversion device (to be referred to as a sensor hereinafter) formed by a plurality of pixels, and arithmetically processing pixel signals output from the sensor. In such case, more accurate focus detection can be attained with increasing level of an image signal defined by the pixel signals. However, when the level of the image signal is too high and exceeds the dynamic range that can process pixel signals, the image signal becomes different from an actual one, thus impairing precision.
Hence, it is a common practice to use an accumulation sensor, and to appropriately control its accumulation time.
When there are a plurality of distance measurement points, the accumulation time of a region corresponding to each distance measurement point is independently controlled. A circuit for appropriately controlling the accumulation time has a large scale, and when the number of distance measurement points is increased, the circuit scale of the sensor including a control circuit is huge. To prevent such problem, the present applicant has proposed a method of controlling the accumulation time using a single controller while dividing a photoelectric conversion element into regions in units of distance measurement points, and sequentially scanning the regions.
However, with this method, since scanning is done all the time during accumulation, many noise components are produced, thus impairing precision. Also, the consumption currents increase, thus wasting energy.
This problem can be solved by slow scanning. However, upon focus detection for a high-luminance object image, the image signal may exceed the dynamic range while scanning in units of regions, and precision may be impaired. Hence, it is hard to attain a small circuit scale and accurate focus detection at the same time.